1. Field of the Invention
This invention is directed toward measurement of density of material, and more particularly directed toward a system for measuring bulk density of material penetrated by a borehole. The system is embodied as a logging-while-drilling gamma ray back scatter density system. The system is configured to minimize the distance between active elements of the downhole logging tool and the borehole environs, to minimize material between source and one or more detectors, to maximize shielding and collimation efficiency, and to increase operational reliability and ruggedness.
2. Background of the Art
Systems utilizing a source of radiation and a radiation detector have been used in the prior art for many years to measure density of material. One class of prior art density measuring systems is commonly referred to as xe2x80x9ctransmissionxe2x80x9d systems. A source of nuclear radiation is positioned on one side of material whose density is to be measured, and a detector which responds to the radiation is positioned on the opposite side. After appropriate system calibration, the intensity of measured radiation can be related to the bulk density of material intervening between the source and the detector. This class of systems is not practical for borehole geometry since the borehole environs sample to be measured surrounds the measuring instrument or borehole xe2x80x9ctoolxe2x80x9d. A second class of prior art density measuring systems is commonly referred to as xe2x80x9cback scatterxe2x80x9d systems. Both a source of nuclear radiation and a detector, which responds to the radiation, are positioned on a common side of material whose density is to be measured. Radiation impinges upon and interacts with the material, and a portion of the impinging radiation is scattered by the material and back into the detector. After appropriate system calibration, the intensity of detected scattered radiation can be related to the bulk density of the material. This class of systems is adaptable to borehole geometry.
Back scatter type systems have been used for decades to measure density of material, such as earth formation, penetrated by a borehole. Typically density is measured as a function of position along the borehole thereby yielding a xe2x80x9clogxe2x80x9d as a function of depth within the borehole. The measuring tool typically comprises a source of radiation and at least one radiation detector, which is axially aligned with the source and typically, mounted within a pressure tight container.
Systems that employ the back scatter configuration with a source of gamma radiation and one or more gamma ray detectors are commonly referred to as xe2x80x9cgamma-gammaxe2x80x9d systems. Sources of gamma radiation are typically isotopic such as cesium-137 (137Cs), which emits gamma radiation with energy of 0.66 million electron volts (MeV) with a half life of 30.17 years. Alternately, cobalt-60 (60Co) is used as a source of 1.11 and 1.33 MeV gamma radiation with a half life of 5.27 years. The one or more gamma ray detectors can comprise ionization type detectors, or alternately scintillation type detectors if greater detector efficiency and delineation of the energy of measured scattered gamma radiation is desired.
The basic operational principles of prior art, gamma-gamma type back scatter density measurement systems are summarized in the following paragraph. For purposes of discussion, it will be assumed that the system is embodied to measure the bulk density of material penetrated by a borehole, which is commonly referred to as a density logging system. It should be understood, however, that other back scatter density sensitive systems are known in the prior art. These systems include tools which use other types of radiation sources such as neutron sources, and other types of radiation detectors such as detectors which respond to neutron radiation or a combination of gamma radiation and neutron radiation.
A back scatter gamma-gamma density logging tool is conveyed along a well borehole penetrating typically earth formation. Means of conveyance can be a wireline and associated surface draw works. This method is used to obtain measurements subsequent to the drilling of the borehole. Means of conveyance can also be a drill string cooperating with a drilling rig. This method is used to obtain measurements while the borehole is being drilled. Gamma radiation from the source impinges upon material surrounding the borehole. This gamma radiation collides with electrons within the earth formation material and loses energy by means of several types of reaction. The most pertinent reaction in density measurement is the Compton scatter reaction. After undergoing typically multiple Compton scatters, a portion of the emitted gamma radiation is scattered back into the tool and detected by the gamma radiation detector. The number of Compton scatter collisions is a function of the electron density of the scattering material. Stated another way, the tool responds to electron density of the scattering earth formation material. Bulk density rather than electron density is usually the parameter of interest. Bulk density and electron density are related as
xcfx81e=xcfx81b(2(xcexa3Zi)/MW)xe2x80x83xe2x80x83(1)
where
xcfx81e=the electron density index;
xcfx81b=the bulk density;
(xcexa3Zi)=the sum of atomic numbers Zi of each element i in a molecule of the material; and
MW=the molecular weight of the molecule of the material.
For most materials within earth formations, the term (2 (xcexa3Zi)/MW) is approximately equal to one. Therefore, electron density index xcfx81e to which the tool responds can be related to bulk density xcfx81b, which is typically the parameter of interest, through the relationship
xcfx81b=Axcfx81e+Bxe2x80x83xe2x80x83(2)
where A and B are measured tool calibration constants. Equation (2) is a relation that accounts for the near linear (and small) change in average Z/A that occurs as material water fraction changes with material porosity, and hence changes with bulk density.
The radial sensitivity of the density measuring system is affected by several factors such as the energy of gamma radiation emitted by the source, the axial spacing between the source and one or more gamma ray detectors, and properties of the borehole and the formation. Formation in the immediate vicinity of the borehole is usually perturbed by the drilling process, and more specifically by drilling fluid that xe2x80x9cinvadesxe2x80x9d the formation in the near borehole region. Furthermore, particulates from the drilling fluid tend to buildup on the borehole wall. This buildup is commonly referred to as xe2x80x9cmudcakexe2x80x9d, and adversely affects the radial sensitivity of the system. Intervening material in a displacement or xe2x80x9cstand offxe2x80x9d of the tool from the borehole wall will adversely affect radial sensitivity of the system. Intervening material in the tool itself between the active elements of the tool and the outer radial surface of the tool will again adversely affect radial tool sensitivity. Typical sources are isotropic in that radiation is emitted with essentially radial symmetry. Flux per unit area decreases as the inverse square of the distance to the source. Radiation per unit area scattered by the formation and back into detectors within the tool also decreases as distance, but not necessarily as the inverse square of the distance. In order to maximize the statistical precision of the measurement, it is desirable to dispose the source and the detector as near as practical the borehole environs, while still maintaining adequate shielding and collimation.
In view of the above discussion, it is of prime importance to maximize the radial depth of investigation of the tool in order to minimize the adverse effects of near borehole conditions. It is also of prime importance to position active elements of the logging system, namely the source and one or more detectors, as near as possible to the outer radial surface of the tool while still maintaining collimation and shielding required for proper tool operation.
Generally speaking, the prior art teaches that an increase in axial spacing between the source and the one or more detectors increases radial depth of investigation. Increasing source to detector spacing, however, requires an increase in source intensity in order to maintain acceptable statistical precision of the measurement. Prior art systems also use multiple axial spaced detectors, and combine the responses of the detectors to xe2x80x9ccancelxe2x80x9d effects of the near borehole region. Depth of investigation can be increased significantly by increasing the energy of the gamma-ray source. This permits deeper radial transport of gamma radiation into the formation. Prior art wireline logging systems use a variety of bow springs and hydraulically operated pad devices to force the active elements of a density logging system against the borehole wall thereby minimizing standoff. Prior art LWD systems use a variety of source and detector geometries to minimize standoff, such as placing a gamma ray source and one or more gamma ray detectors within stabilizer fins that radiate outward from a drill collar. This also tends to minimize intervening material within the tool, and position source and detectors near the borehole environs, but often at the expense of decreasing the efficiency of shielding and collimation. Furthermore, this approach introduces certain operational problems in that harsh drilling conditions can break away stabilizer fins resulting in the loss of the instrument, and more critical the loss of a radioactive source, in the borehole. Yet other prior LWD systems dispose a source and one or more detectors within a drill collar with a stabilizer disposed between source and detectors and the borehole and formation. This is more robust operationally, but the amount of intervening material between active tool elements and the borehole environs is increased. Distance between the source and detectors, and the surrounding borehoke environs, is also not minimized.
This invention is directed toward a logging-while-drilling (LWD) gamma ray back scatter density system wherein elements are configured to place a sensor preferably comprising a source and one or more detectors as near as practical to the borehole environs, to maximize shielding and collimation efficiency, and to increase operational reliability and ruggedness. It should be understood, however, that the basic concepts of the invention can be employed in other types and classes of LWD logging systems. As an example, concepts of the invention can be used in a neutron porosity system for measuring formation porosity, wherein the sensor comprises a neutron source and one or more neutron detectors. As another example, concepts of the invention can be used in natural gamma radiation system for measuring shale content and other formation properties, wherein the sensor comprises one or more gamma ray detectors. Basic concepts of the system can be used in other classes of LWD logging systems including electromagnetic and acoustic systems.
The tool element of the LWD system is conveyed by a drill string along the borehole penetrating an earth formation. A drill bit terminates the drill string. The drill string is operated by a standard rotary drilling rig, which is well known in the art. The LWD tool comprises three major elements. The major first element is a drill collar with an axial passage through which drilling fluid flows, and which also contains a cavity within the collar wall and opening to the outer surface of the collar. The second major element is an instrument package that is disposed within the cavity and which protrudes radially outward from the outer surface of the collar. The third major element is a stabilized, which is disposed circumferencially around the outer collar surface. An axial alignment channel is formed on the inner surface of the stabilizer and is sized to receive the protruding portion of the instrument package.
The system is preferably embodied as a gamma-gamma density logging system, although basic concepts of the invention can be used in other types or classes of LWD systems. The instrument package comprises a source of gamma radiation and one or more gamma ray detectors. Two detectors are preferred so that previously discussed data processing methods, such as the xe2x80x9cspine and ribxe2x80x9d method, can be used to minimize adverse effects of the near borehole environment. The source is preferably cesium-137 (137Cs) which emits gamma radiation with an energy of 0.66 million electron volts (MeV). Alternately, cobalt-60 (60Co) emitting gamma radiation at 1.11 and 1.33 MeV can be used as source material. The source is affixed to a source holder that is mounted in directly into shielding in the instrument package rather than mounting into or through the collar as in prior art systems. This source mounting offers various mechanical, operational and technical advantages as will be discussed subsequently. The detectors are preferably scintillation type such as sodium iodide or bismuth germinate to maximize detector efficiency for a given detector size.
The instrument package framework is fabricated with a high atomic number material, commonly referred to as xe2x80x9chigh Zxe2x80x9d material. High Z material is an efficient attenuator of gamma radiation, and permits the efficient shielding, collimation and optimum disposition of the source and detectors with respect to the borehole environs. A pathway in the high Z instrument package leading from the source to the stabilizer forms a source collimator window. The source collimator window is filled with a material that is relatively transparent to gamma radiation. Such material is commonly known as a xe2x80x9clow Zxe2x80x9d material, and includes materials such as a ceramic, plastics and epoxies. The axis of the source collimator window is in a plane defined by the major axis of the collar and the radial center of the instrument package. Pathways in the instrument package leading from each detector to the stabilizer form detector collimator windows. Again, axes of the detector collimator windows are in the plane defined by the major axis of the collar and the radial center of the instrument package, and the windows are filled with low Z material. The stabilizer comprises windows over the collimator windows that are fabricated with low Z material and, therefore, are also relatively transparent to gamma radiation. Power supplies and electronic circuitry, used to power and operate the detectors, are preferably remote from the instrument package.
The instrument package is disposed within the cavity in the drill collar, with the protruding portion fitting within the axial alignment channel of the surrounding stabilizer. The instrument package is preferably removably disposed within the cavity using threaded fasteners or the like. This arrangement permits relatively easy replacement of the entire instrument package in the event of malfunction thereby increasing operational efficiency. Because a portion of the instrument package is positioned within the alignment channel, source and detector elements are moved radially outward thereby minimizing the distance between these elements and the borehole environments. This, in turn, reduces the amount of intervening material between these elements therefore making the system more responsive to the borehole environs. Furthermore, this geometrical arrangement maximizes the gamma ray flux per unit area entering the borehole environs, and also maximizes the flux per unit area of gamma radiation returning to the detectors. The source is preferably mounted in the instrument package by threading into a small, mechanically suitable insert disposed within the instrument package shielding material. This arrangement yields maximum radial shielding and collimation of the source, even though design criteria discussed above minimize radial spacing between the source and the borehole environs. A substantial portion of the instrument section, including the gamma ray source, is preferably disposed in the cavity within the collar. This design produces a physically robust system, wherein the loss of the source would be minimized in the event that stabilizer protrusions were lost during the drilling operation. For an instrument package with fixed dimensions, the gamma ray source may be disposed outside of the cavity when collars of relatively small diameter are used.